This invention relates generally to ceramics and more particularly to a process for making ceramic forming titanate material such as barium strontium titanate having optimum electrical properties and a product made by such process.
Various titanate ceramic materials are useful in the electronics industry due to their electrical properties. Material having a high dielectric constant, for example, is useful in making capacitors. Other desirable characteristics relate to the pyroelectric coefficient for materials used, for example, as infra red detectors and materials having a steep sloped positive coefficient of resistivity for use as thermal sensors. Typically, such ceramic forming materials, provided in powdered form, are combined with an organic binder and pressed into a desired configuration and sintered to form a ceramic. For many devices it is important to use a powder which has a very small, uniform particle size and which is very dense so that the powder can be formed into very small and precise configurations. By way of example in microminiature heat sensing devices, material used as infra red (IR) detectors may be formed into a large number of pixels, in the order of several thousand per square centimeter. A dense powder formed of small, uniform particles used in making such configurations is needed to avoid occlusions, voids and the like.
Barium strontium titanate powder made for IR detectors has been made by direct pyrolysis of a common solution of precursor materials including barium acetate, strontium acetate, tetra-isopropyl titanate, lactic acid and water of approximately 10-15% product by weight using the teaching of U.S. Pat. No. 3,637,531. The common solution can also be provided by substituting titanium ammonium lactate for tetra-isopropyl titanate and lactic acid. This method for making the material has the advantage of enabling precise stoichiometric control and therefore obtaining optimum electrical properties for the particular composition in question. Another advantage provided by this process is that compositional modifications in the ratio of barium/strontium, partial substitution of ions such Ca, Pb or Zr, and addition of dopants such as Mn, Fe, Co, Y, Nb, etc., are easily achieved since these constituents can be readily dissolved and combined in the common solution.
Direct calcination of such a solution, however, results in the formation of tenaciously bound aggregates in the final powder, even after intensive milling as in a tungsten carbide ring mill. These aggregates lead to defects in the final ceramic, for example, tiny areas of low density. When the material is intended for use, for example, as infra red detectors, this can result in missing or poorly formed pixels. Even in applications where such tiny defects are not as critical the milling process represents an undesirable expenditure of time and energy.
It is an object of the present invention to provide an improved method for producing powder suitable for making ceramic material and the provision of material both in powder and coated form, produced by such method.